The present invention relates to measurements involving very small currents and, in particular, to a preamplifier for use with a source measure unit.
Source measure units are used to make precision measurements in many fields, including the testing of semiconductor products. For example, U.S. Pat. No. 5,039,934 describes one such device and range-changing in such a device is described in U.S. Pat. No. 5,144,154, both of which are incorporated herein by reference. Source measure units force either a voltage or a current and then respectively measure a resulting current or voltage.
It is not uncommon for these devices to measure currents of a few picoamperes. The measurement of such low currents is adversely affected by undesired capacitances and other effects such as dielectric absorption of charge carriers. These effects are exacerbated by multiple source measure units, long test cables and relay switching matrixes used in making such measurements in a production environment, such as found in a semiconductor fabrication facility, where many measurements must be made as quickly as possible.
Switching matrixes allow source measure units to be selectively connected to various test points on a device under test. One such switching matrix is described in U.S. Pat. No. 5,644,115 and is incorporated herein by reference.